Such system is known from U.S. Pat. No. 4,876,600. This prior art reference discloses an apparatus to produce a composite image from information supplied by a plurality of independently and mutually asynchronously operating sources, such as TV cameras, mass storage for digital image data and raster graphic generators. The sources provide the information to a plurality of dual-ported buffers. The data stored in the buffers are read in a synchronized manner. A pre-programmed mask storage selects on a pixel-by-pixel basis which one of the buffers is to supply the information for a given pixel of the composite image. The mask storage furnishes control data words that each comprise a number of bits equal to that of the number of sources to be handled. That is, the data width which may be read in parallel corresponds exactly to the number of sources. One bit is set per data word to achieve this selectivity feature.
The prior art reference does not address the problem of signals that are mutually differently formatted. Generally, video signals occur in an analog representation, such as certain TV signals, or in a digital representation such as computer-generated graphics. In addition, each such representation may have its information contents encoded in a variety of ways, dependent on the signal's intended use (display, editing, storage, transmission, etc.). Following video signal types are mentioned here by way of example: compressed still images (JPEG), uncompressed still images, compressed moving images (MPEG), uncompressed moving images (PAL, NTSC, SECAM, D2MAC, HDMAC, YUV, Y/C, RGB, RS170A, RS330), video patterns, etc. The video signals and the graphics are necessarily reformatted in a common format before they can be simultaneously displayed on a same display, e.g., a PC's monitor.